FIG. 10 shows a laser processing machine having a laser beam branching apparatus based on the conventional technology. This laser processing machine has two machining heads 1a and 1b, and laser beams are given to the machining heads 1a and 1b respectively from one unit of laser oscillator 2.
The laser oscillator 2 has an excitation light source 3, a solid state laser element 4 as an excitation medium, a cavity 5 incorporating therein the excitation light source 3 and the solid state laser element 4, and an optical resonator 8 comprising a total reflection mirror 6 and a partial reflection mirror (output mirror) 7 provided opposite to each other in both side of the cavity 5, and a laser beam L is outputted from the partial reflection mirror 7 to a laser optical system.
The laser optical system has a laser beam branching apparatus 9 for selectively branching the laser beam L outputted from the optical resonator 8 into two light paths, magnifying lenses 10a, 10b provided in parallel to each other at midpoints in the branched two light paths (parallel light paths) respectively, collimator lenses 11a, 11b, and converging lenses 12a, 12b, and laser beams La, Lb in the two light paths are propagated to the machining heads 1a, 1b through optical fibers 13a, 13b respectively.
Provided on each of incident end faces of the optical fibers 13a, 13b are optical fiber incident beam converging lenses 14a, 14b, and laser beams La, Lb converged by the converging lenses 12a, 12b are given to the optical fiber incident beam converging lenses 14a, 14b respectively.
It should be noted that converging lens systems each comprising a collimator lens 15a (15b) and a converging lens 16a (16b) are provided in the machining heads 1a, 1b respectively, and the laser beams La, Lb are converged to nozzle sections 17a, 17b provided at tips of the machining heads 1a, 1b, respectively.
The laser beam branching apparatus 9 has a branching mirror holder 21 holding the branching mirror (partial reflection mirror) 19 and enabling pivotal displacement of 180 degrees at a center axial line of a pivot 20 as well as a transfer mirror (total reflection mirror) 22 located and fixed in the light path of the laser beam Lb at a slope of 45 degrees.
The branching mirror 19 is positioned at either a position A (a beam branching position) located in a light path between the laser beams L and La (an optical axis of the laser beam L and that of the laser beam La are an identical axial line) at a slope of 45 degrees or a position B (an escaping position) pivotally displaced from the position A by 180 degrees and escaping outside the light paths between the laser beams L and La according to rotation by 180 degrees of a branching mirror holder 21 around a central axis of a pivot 20, and at the position A the branching mirror branches the laser beam L into the laser beams La, Lb directing to the machining heads 1a and 1b respectively, and stops branching of the laser beam at the position B so that only the laser beam La is allowed to go to the machining head 1a.
As the other type of laser beam branching apparatus, there is the one as disclosed in Japanese Patent Laid-Open Publication No. HEI 7-294833, in which total reflection mirrors each having a form like a right-angled triangle located opposite to each other in a main laser beam light path are provided movably in parallel to each other in the direction of crossing the optical axis at right angles to the main laser beam optical axis, the branching mirror branches the main laser beam into two beams by refracting the main beam to both sides of the mirror by 90 degrees in opposite directions when an apex (ridgeline) of the total reflection mirror having a form like a right-angled triangle is positioned on the optical axis (central position) of the main laser beam, and the main laser beam is refracted only to one side by 90 degrees when the total reflection mirror having a form like a right-angled triangle is located where the main laser beam is introduced into only one of slants of the mirror and the laser beam is not branched.
As shown in FIG. 10, in an apparatus in which a light path is switched (branched) to the other by pivotally displacing the branching mirror holder 21 holding only one piece of the branching mirror 19, when there occurs angle displacement in the branching mirror holder 21 at the position A, an incident angle of the laser beam into the transfer mirror 22 is changed and a position of the incident light into an optical system to be converged to the optical fiber 13b is also displaced, and for this reason converging performance of the laser beam Lb outputted from the optical fiber 13b is reduced, and at the worst the laser beam Lb is not introduced into the optical fiber 13b, which sometimes causes the optical fiber 13b to be burnt and resultantly processing to be impossible.
For this reason, in order to prevent generation of angle displacement in the branching mirror holder 21, a driving system of the branching mirror holder 21 for realizing selection of a light path is required to have high-angular positioning accuracy, which causes the structure to be complicated and so much cost to be required.
In the laser beam branching apparatus as shown in Japanese Patent Laid-Open Publication No. HEI 7-294833, the above problem does not occur, but the laser beam is branched at the optical axis (beam center) of the main laser beam as an interface therebetween, so that a position of maximum light intensity of the branched laser beam is not located at the center of the beam, and the light intensity is distributed on one side, and for this reason a high-quality branched laser beam can not be obtained.